Yesterday the DHS Infrastructure Security Compliance
Division (ISCD) updated responses to two Frequently Asked Questions (FAQ) on
the Chemical Facility Anti-Terrorism Standards (CFATS) Knowledge Center web page. Both
questions deal with calculating screening threshold quantities for mixtures of
flammables (one is specifically for propane).
The two revised FAQs are:
#1566 How
does a facility calculate the Screening Threshold Quantity (STQ) for propane in
a mixture?
Flammable Mixtures
The response for FAQ #1373 was completely re-written with a
completely different process from the original description.
The original response said essentially that when COI were in
a mixture at more than 1% the entire weight of the COI was reported as the COI
with the highest concentration; the facility did not report the weight of the
other COI in that mixture. The exception to that was propane; if propane was in
the mixture at less than 87.5% the next greatest COI in that mixture was
reported, not the propane.
The new response also requires the checking of the NFPA
flammability rating of the mixture. If the NFPA rating is 4 then the rule
described above applies. If the NFPA rating is 1, 2 or 3 (and not a fuel), then
only the actual weight of each COI in the mixture (present at 1% or more) would
be reported.
Interestingly, no mention is made of propane in the new FAQ
response.
Propane Mixtures
A relatively minor modification was made to the response to
FAQ #1566. The second paragraph was expanded to provide more of the information
from the FAQ response above for mixtures that contain less than 87.5% propane.
It addresses the case where the NFPA rating of the propane containing mixture is
4. This response does not address the situation where the NFPA rating is 1, 2,
or 3.
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